1. Field of the Invention
This invention relates to the catalytic steam reforming of hydrocarbons in the production of methane-containing gases, such as Substitute Natural Gas and to catalysts for use in such steam reforming operations. More particularly, the invention relates to the catalytic steam reforming of the heavier hydrocarbon factions and to new catalysts for use in such steam reforming.
2. Description of the Prior Art
The catalytic steam reforming of hydrocarbons for the production of methane-containing gases, such as Town Gas or Substitute Natural Gas (SNG), has been known for many years. For example, in our prior U.K. Patent Specification No. 820,257 there is described and claimed a process for the production of methane-rich gases wherein steam and hydrocarbons are reacted in the presence of a catalyst comprising nickel and alumina to produce a methane-rich gas.
This catalytic steam reforming process has been further developed as described, for example, in our U.K. Patent Specification Nos. 969,637, 994,278, 1,152,009, 1,150,066, 1,155,843 and 1,265,481. However, although these modified processes are themselves very efficient, the governing constraint is the problem of maintaining catalyst life under the reforming conditions.
It has been recognised that such catalysts may be subjected to sintering at high temperatures. The sintering process is characterised by a decrease in both metal and total surface area and results in a loss of catalytic activity. The effect of steam on the sintering of alumina-based catalysts during the steam reforming of hydrocarbons has been reported in "Journal of Catalysis," Vol. 24, 2 Feb. 1972, pages 352-355. Sintering in the presence of steam is much more rapid at comparable temperatures than in air, and a different mechanism is probably involved.
It has been suggested that the vital step in the sintering process is the conversion of the metastable .gamma.-alumina support, which has a relatively high surface area, into the more stable .alpha.-alumina form (corundum), having a very low surface area. It has been observed that particles of .gamma.-alumina begin to grow during catalyst operation. For example, after a sintering test in steam at 600.degree. C. the crystallite size of the .gamma.-alumina is typically 90-120 A, whereas immediately after reduction of the catalyst, the crystallite size is in the range of 60-70 A. At the same time, some growth of the nickel particles occurs. It is, however, predominantly the conversion of .gamma. to .alpha.-alumina which causes massive changes in the structure of the catalyst and triggers the irreversible deactivation of the catalyst.
Examination of used coprecipitated nickel-alumina catalysts shows that the .alpha.-alumina particles formed are over 1000 A in size. Hence, the formation of a single .alpha.-alumina particle required many .gamma.-alumina ones and must involve macroscopic rearrangement of the structure of the catalyst. The alumina particles are then no longer able to keep the nickel crystallites apart. Serious nickel sintering is therefore triggered by the start of corundum formation.
In other alumina systems, it has been recognised that thermal sintering may be prevented by the addition of other metals to the alumina lattice. The .gamma. to .alpha.-alumina phase change involves the conversion of the cubic close packed oxygen ions of the spinel-like .gamma.-structure into the hexagonal close packed array of .alpha.-alumina. In pure alumina, the conversion occurs thermally at about 1000.degree. C. Many studies have been undertaken to investigate the effect of small concentrations of other metal ions upon the temperature and rate of .gamma. to .alpha.-alumina phase change and one has been to study the effect of chromium on the rate of formation of .alpha.-alumina at 1100.degree. C. (G. C. Bye and G. T. Simkin, J. Am. Cer. Soc. 57, (8), 367, (1974)). These workers have concluded that from 2 to 4% weight additions of chromium decreases the rate of conversion at that temperature and that the linear relationship between the surface area loss and the .alpha.-alumina formation is only slightly affected by the addition.